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多线程的那点儿事(之读写锁)

2011-12-05

【 声明:版权所有,欢迎转载,请勿用于商业用途。 联系信箱:feixiaoxing @163.com】 在编写多线程的时候,有一种情况是十分常见的。那就是,有些公共数据修改的机会比较少。相比较改写,它们读的机会反而高...

【 声明:版权所有,欢迎转载,请勿用于商业用途。 联系信箱:feixiaoxing @163.com】

在编写多线程的时候,有一种情况是十分常见的。那就是,有些公共数据修改的机会比较少。相比较改写,它们读的机会反而高的多。通常而言,在读的过程中,往往伴随着查找的操作,中间耗时很长。给这种代码段加锁,会极大地降低我们程序的效率。那么有没有一种方法,可以专门处理这种多读少写的情况呢?

有,那就是读写锁。

(1)首先,我们定义一下基本的数据结构。

typedef struct _RWLock

{

int count;

int state;

HANDLE hRead;

HANDLE hWrite;

}RWLock;

typedef struct _RWLock

{

int count;

int state;

HANDLE hRead;

HANDLE hWrite;

}RWLock; 同时,为了判断当前的锁是处于读状态,还是写状态,我们要定义一个枚举量,

typedef enum

{

STATE_EMPTY = 0,

STATE_READ,

STATE_WRITE

};

typedef enum

{

STATE_EMPTY = 0,

STATE_READ,

STATE_WRITE

}; (2)初始化数据结构

RWLock* create_read_write_lock(HANDLE hRead, HANDLE hWrite)

{

RWLock* pRwLock = NULL;

assert(NULL != hRead && NULL != hWrite);

pRwLock = (RWLock*)malloc(sizeof(RWLock));

pRwLock->hRead = hRead;

pRwLock->hWrite = hWrite;

pRwLock->count = 0;

pRwLock->state = STATE_EMPTY;

return pRwLock;

}

RWLock* create_read_write_lock(HANDLE hRead, HANDLE hWrite)

{

RWLock* pRwLock = NULL;

assert(NULL != hRead && NULL != hWrite);

pRwLock = (RWLock*)malloc(sizeof(RWLock));

pRwLock->hRead = hRead;

pRwLock->hWrite = hWrite;

pRwLock->count = 0;

pRwLock->state = STATE_EMPTY;

return pRwLock;

} (3)获取读锁

void read_lock(RWLock* pRwLock)

{

assert(NULL != pRwLock);

WaitForSingleObject(pRwLock->hRead, INFINITE);

pRwLock->counnt ++;

if(1 == pRwLock->count){

WaitForSingleObject(pRwLock->hWrite, INFINITE);

pRwLock->state = STATE_READ;

}

ReleaseMutex(pRwLock->hRead);

}

void read_lock(RWLock* pRwLock)

{

assert(NULL != pRwLock);

WaitForSingleObject(pRwLock->hRead, INFINITE);

pRwLock->counnt ++;

if(1 == pRwLock->count){

WaitForSingleObject(pRwLock->hWrite, INFINITE);

pRwLock->state = STATE_READ;

}

ReleaseMutex(pRwLock->hRead);

} (4)获取写锁

void write_lock(RWLock* pRwLock)

{

assert(NULL != pRwLock);

WaitForSingleObject(pRwLock->hWrite, INFINITE);

pRwLock->state = STATE_WRITE;

}

void write_lock(RWLock* pRwLock)

{

assert(NULL != pRwLock);

WaitForSingleObject(pRwLock->hWrite, INFINITE);

pRwLock->state = STATE_WRITE;

} (5)释放读写锁

void read_write_unlock(RWLock* pRwLock)

{

assert(NULL != pRwLock);

if(STATE_READ == pRwLock->state){

WaitForSingleObject(pRwLock->hRead, INFINITE);

pRwLock->count --;

if(0 == pRwLock->count){

pRwLock->state = STATE_EMPTY;

ReleaseMutex(pRwLock->hWrite);

}

ReleaseMutex(pRwLock->hRead);

}else{

pRwLock->state = STATE_EMPTY;

ReleaseMutex(pRwLock->hWrite);

}

return;

}

void read_write_unlock(RWLock* pRwLock)

{

assert(NULL != pRwLock);

if(STATE_READ == pRwLock->state){

WaitForSingleObject(pRwLock->hRead, INFINITE);

pRwLock->count --;

if(0 == pRwLock->count){

pRwLock->state = STATE_EMPTY;

ReleaseMutex(pRwLock->hWrite);

}

ReleaseMutex(pRwLock->hRead);

}else{

pRwLock->state = STATE_EMPTY;

ReleaseMutex(pRwLock->hWrite);

}

return;

}

文章总结:

(1)读写锁的优势只有在多读少写、代码段运行时间长这两个条件下才会效率达到最大化;

(2)任何公共数据的修改都必须在锁里面完成;

(3)读写锁有自己的应用场所,选择合适的应用环境十分重要;

(4)编写读写锁很容易出错,朋友们应该多加练习;

(5)读锁和写锁一定要分开使用,否则达不到效果

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